When the project of building a very small transceiver was accomplished 4 years ago, I still lacked lots of skills in setting up electronic circuits using SMD technology. The radio’s craftmanship had been a defective to a certain degree (there were still lots of things to learn when using SMDs on Veroboards), the inside looked more or less “messy” and the performance was not sited in the premium league. Particularly the receiver was prone to IMD problems when signals on the band were strong. But because I liked the outer appearance of the radio a total revision of the inside had to be performed.
The major changes that were used to improve the radio are:
Usage of a Si5351 clock oscillator as VFO and LO instead of an Xtal controlled LO and AD9835 as VFO,
Only one SSB filter (commercially made) instead of 2 ladder filters,
Dual-Gate MOSFET as 1st mixer (instead of SA612),
2SC2078 as push-pull pair in the final TX stage,
All SMD components are now mounted to the underside of the board,
TBA820M instead of bipolar equipped push-pull audio amplifier,
Cabinet size has been enlarged slightly (about plus 0.5 cm in length),
Proper cabling instead of “spaghetti” arrangement,
Copper band has been used to improve radio frequency grounding.
Things that were not changed are frequency layout (14 MHz), the ATMega328P microcontroller (MCU) and cabinet size etc.
Even if the changes to the previous version are minor, I had to revise the schematic nearly completely (High resolution schematic):
The radio consists of 3 sections:
Control unit (MCU, Si5351 oscillator, 1306 OLED and related stuff
In the receiver I changed the NE612 into a dual gate MOSFET mixer stage because I found out that the IMD3 was causing problems in the evenings when high signal levels were present. The dual gate MOSFET mixer turned out to be more stable in respect to high signal levels. With the Si5351 being able to produce about 3 Vpp. of rf energy the mixer could be fed with an appropriate signal level.
The MC1350 had been removed because the simplicity of the AGC that section that could also be simplified because only one type of AGC voltage had to be produced. Remember: The dual gate MOSFET and the MC1350 have reverse AGC characteristics, thus an AGC that controls both types of amplifiers has to produce two types of AGC voltage. One rising and one falling when signal levels increase in the receiver.
The microphone amplifier was not necessary because an electret microphone outputs enough audio frequency voltage to drive the NE612 mixer directly. An intermediate amplifier with bipolar transistor amplifies between the SSB filter and the TX mixer pushed the signal, thus enough energy always is present in the first transmitter stages.
The remaining transmitter has been not changed, only the final amplifier transistors have been replaced with a pair of 2SC2078 (2SC1957 in previous version). Transmit power is now 6 watts (when DC is 13.2 volts from my QRP battery package).
Output spectrum is as follows (Pout = 5W PEP)
Based on a discussion with WA2MZE here on my blog I tried to minimize physical expansions of a P-channel MOSFET based T/R switch. The basic design can be found here, only two P-channel switching MOSFETs are used.
The circuit is so simple, it fits on a piece of Veroboard just 1 square centimeter in size and put into a piece of heat shrink tubing. After connected to the 12V system it was stored behind the front panel:
The inside has also been straightened (please, don’t say its is still messy! 😉 ):
Under the Si5351 breakout the audio amp is hidden. I think available space has been used to the maximum and component density of the board is OK. 😉
Here a view to the underside where all the small SMD components have been placed:
Front panel labeling
Times are getting harder because I’m running short in these adhesive letters that are not available today anymore. An alternative had to be found. Initial tests with the so called “toner transfer method” had been frustrating, but I have found the idea to use labels for laser printers that are cheap and allow individual front panel design.
Here the steps in brief to get a first class front panel labeling:
Step 1: Buy self-adhesive transparency film for laser printers.
Step 2: Scan your front panel using a flat bed scanner:
Step 3: Cut the image and work out your front panel. Then enhance the borders of the items you want to label later:
Step 4: Eliminate the background by using the “cut” tool:
Step 5: Put the labels into the right places and later cut out the borders of the items you have just labelled:
Step 6: Now you are nearly ready to print but one step must be done: Measure the size of your front panel and bring the picture exactly to this size. If you are lucky (like I was) the picture is in the right dimensions. If you are not, you can copy the picture to a text processing software and adjust the size of picture exactly. Make 4 or 5 five copies on the same sheet and print it out with your laser printer.
Step 7: Clean your front board with grain alcohol and fix one copy of the laser print taking the precise position of the label.
Step 8: Cut the holes and other culverts with a sharp cutter knife or scalpel.
Step 9: Be happy because of having made a top quality front panel!
Like its predecessor the radio has been mounted into a U-shaped frame of aluminum. Height is 3 centimeters, thickness of the sheet metal is 1 mm. The front panel has been attached with angle plates also made from alu and fixed with M2 screws. This makes a rugged mounting frame for the veroboard and the additional mechanical structures like sockets for antenna, DC supply and headphone.
To finish the cabinet, a base and a top cover from 0.5 mm aluminum sheets have been bent exactly. Precision is now improved because I started a new method: Before bending the sheet metal I cut a wooden block using a precise buzz saw. In this case case exactly 74 millimeters wide (7 centimeters from the inside, plus 2×1 millimeters for the thickness of the mounting frame and another 2 millimeters of space you need because of the minimum bending radius that is required for the metal sheet. Using this method the cover exactly fits onto the mounting frame.
So, that’s the story of another revision of my radios. Thanks for watching!
Sorry for having deferred the description of the transmitter. The recent days I have been concerned with a new frequency layout for the transceiver. I found that the 17m-band could be an interesting topic because when tuning on internet based SDR pages the last days I saw many strong signals appearing. This might be due to the fact that sun is higher now in the northern hemisphere and conditions will even be better with solar cycle #25 now about being to commence.
Based on these considerations I changed the band plan for the 5-band radio: 10m band has been removed, instead 17m has been added.
The new band layout now is 80m-40m-20m-17m-15m.
Here are the respective values for coils installed into the band pass filters (BPF) and the layout for the final low pass filter (LPF).
Hint: Inductance for the BPF coils have been measured with (probably) excess error ratio. Thus calculations are resulting in a different resonant frequency for the LCs when using Thompson’s formula!
Currently some additional tests with the the transmitter are pending, but full description will follow the next days. So, stay tuned! 😉
This is some sort like “Copy & Paste”, a useful mean if you want to create a doctorate, like the former German Minister of Defense Mr Guttenberg once did. 😉 I don’t want to achieve a doctorate but the receiver of this radio is more or less the same I have constructed for the Midi6-transcevier. So I just copied the schematics and put down the changes in this paper.
To see a full sized picture of the RECEIVER, please click here!
Starting the tour on the left you can see the band switch unit, beginning with a BCD decoder that converts a 3-bit pattern created by the MCU into a 5 line decimal output. The ULN2003 then is a driver designed for motor controls but it is very useful as a relay driver as well. Integrated clamp diodes and open collector circuit make it practical as a driver circuit for this unit.
Next is the band pass filter section. I still use relays for switching the respective filter because I found that it is the best way to keep unwanted signals low from passing the filter, provided you use relays that can serve this purpose., Here signal relays TQ2-12V by Panasonic have been applied. Coils are small TOKO style coil formers with 5.1 mm (2×2.54mm i. e. 2×0.1″) pin spacing.
RF preamp is equipped with a dual gate MOSFET like the BF900 or so. The “AGC” this time is to be manually, just connect the AGC input (which now is an “MGC” to say it correctly!) of the stage to a 10kOhm variable resistor allowing a voltage swing between 0 and 12 V and this will lead to a preamp stage with gain control in the range of 25dB. This variable resistor is to mounted into the front panel, just to be concise.
The receiver’s mixer is an SL6440 which has great IMD3 performance (about 30dB) and has been used instead of diode ring mixer. Some dBs of gain are achieved as well but not the amount you can expect from an SA602.
In practical terms the ic really proves what the manufacturer promises. On 40m e. g. with a large doublet antenna no IMD products are audible even when strong broadcast station are next to the amateur radio band. A really worthy trial with this receiver!
Due to the fact that the following SSB filter is used for the transmitter also, another signal relay switches the filter between the receiver and the transmitter branch.
Next the MC1350 video amp is installed to do the major amplification with the interfrequency signal. It is gain controlled by the AGC circuit on the right side of the schematic. Gain is minimum if AGC input is around 7V or higher.
The product detector is a dual gate MOSFET which is only there because this one has a slight amount of gain and does not consume much space on the tiny boards.
The audio preamp stage is also very simple, just a bipolar transistor with negative feedback applied via a large resistor (390k) also biassing the unit to an appropriate value.
The audio main amp here is not an ic (like the inevitable LM386 e. g.) but it is a push-pull arrangement using 3 bipolar transistors. The stage that enhances the voltage is designed with a BC547, the stage that is bound for current amplification uses a pair of complementary transistors (BD137 -NPN- and BD 138 -PNP-). Audio power is about 1 Watt which is suffice for a small radio.
AGC uses an operational amplifier, any type like the LM358 will work great. The LM358 contains two identical amplifier stages. The first is used to bring the audio signal to a certain level, then rectifying this voltage and subsequently bringing it into a time constant consisting of a charged capacity (2.2uF) and a discharging resistor (3.3M), The circuit has very fast response, so there is no annoying “plopp” when a strong signal breaks in) and the decay is very soft.
The second stage just works as an instrumentation amplifier putting out up to 12V to control the input of the MC1350 at PIN5.
To end this article let’s have a look at the practical setup of the receiver:
A compact SSB transmitter/receiver will be presented. This unit covers 5 bands within the amateur radio spectrum (3.5, 7, 14, 21 and 28 MHz). Receiver is a single conversion unit with an interfrequency of 9 MHz. Transmitter uses 5 stages and has got a power level of 10 watts PEP output.
Frequency generation is done by integrated ready made modules like an AD9850 as VFO, and an Si5351 as LO. Microcontroller is an Arduino Pro mini AtMega328 driving a colored TFT LCD with ST7735 chipset.
The whole device has been constructed in SMD but can also be setup by using “thru hole” techniques or mixed installations.
The unit is built into into a mounting frame of aluminum sheets of standardized width. Size of the whole radio is 17 x 12 x 5 centimeters. It is, to a certain degree, the “Little Brother” of the “Midi6“-Transceiver that had been designed mainly for experimental purposes.
Multiband QRP transceiver projects are a challenging undertaking for the radioamateur. The even more challenging matter is to build it as neat as possible.
The “Midi6” transceiver has been an interesting step which made me learn a lot of things. But it is a much too bulky for my needs (producing compact and lightweight portable gear for traveling, hiking etc. ) On the other hand I found that I don’t really need 160m installed in the radio (due to antenna problems here at my site) which defined the next multibander having a “classical” (i. e. 70s) layout with 80, 40, 20, 15 and 10 meters.
An important point was to use ready made modules or breakout boards for the major digital and analog circuits:
First I thought about using the Si5351 as VFO and LO because it contains 3 oscillators on one chip. But I gave that idea away very fast because there were to many spurious signals and the thus the receiver had to many “birdies” which I don’t accept. Having had some of the Chinese made AD9850 boards still here on the shelf I gave that one a try and was finally relatively happy with receiver performance.
The microntroller and its application also has been a challenge because for a multiband transceiver an Arduino Pro Mini might be a little bit weak because the number of ports is very limited. But it finally worked out when planning is carefully done and optimizing is brought to its limits. The port usage is as follows:
ISP leads are used for controlling the DDS and for uploading the software to the controller. This is done because the inputs of the DDS are high Z inputs that do not affect the ISP data transfer. On the other hand the programmer goes to high Z if there is no data to be sent to the controller. Thus testing the radio is possible when programming leads are connected.
LCD is an ST7735 TFT colored display because I found the OLEDs with 1306 and 1106 drivers to noisy on the higher bands where band noise is weak and therefore digital noise produced in the radio comes more into the foreground. And, above all, a colored display makes much more impression than an ordinary b/w one. 😉
Mechanical construction and transceiver units
For this radio I ordered aluminum strips holding a width of 5 centimeters via ebay. Thickness is 1.5 mm. From this material a very rugged frame has been constructed that gives the whole rig a very good mechanical stability.
Major units in this construction
The rig is very much unitized, each functional of a module section is soldered to a very small piece of veroboard that has been cut out from a larger piece of material. It is fixed to the aluminum basis by using inserted nuts with M2 screw thread. The main advantage is: If one unit fails it is easy to reconstruct it and put it to the place the predecessor has been mounted and second grounding is excellent because the small single units don’t require long grounding leads because the boards are very small in size and the 4 corners all have ground potential. Particularly for the transmitter I can say that I had never any unwanted oscillations.
The transmitter is 100% stable on all the 5 bands, which was not the way with the first “Gimme 5”-Transceiver that had severe layout problems in the transmitter having the initial BPFs very close to the final rf power stage. But in the end you should be knowing more than in the beginning pf a project. So is true here. 😉
The picture shows a close-up of the receiver section that consists of 5 single units (from the left)
Dual-gate MOSFET preamplifier (in the picture veiled by shielded cables) and rx mixer (SL6440)
interfrequency amplifier (MC1350) and product detector (dual gate MOSFET)
audio preamp (BC547) and main amp (3 transistors, the 2 finals in push-pull circuit)
AGC with OP (LM358) and bipolar transistors as voltage regulators.
The same technique has been used for the transmitter:
Starting from the left you notice an SSM2166 microphone compressor ic by Analog Device which also is the main microphone amplifier. Next is an AN612 mixer as DSB generator, followed by an NE612 serving as transmit mixer.
The second board from the right is a 3 stage unit to bring the transmit signal to a power level of about 150mW (Dual gate MOSFET, 2N2222 and 2SC2314 as active semiconductors in this order). On the right a push-pull stage equipped with 2 2SC2078 and relatively high emitter degeneration (2 Ohms for each transistor) brings the power up to 500mW.
Transmitter gain can be controlled with an MCP4725 DAC that is set for each band individually and helps much to compensate gain increase on the lower bands. This DAC is also connected to the microcontroller’s I²C-bus and data for each band is saved in EEPROM and is being recalled if a certain band is switched.
Tha main amp is centered on the center side of the mainframe:
On the left side of the tx pa unit there are 2 power transistors (2SC1969 by eleflow) mounted to a small strip of 3mm thick aluminum that is connected to another much thicker block of Al. Here a large heatsink can be mounted when the device is under test or finally fixed into the cabinet when using the aluminum cabinet as heatsink. Connected to the aluminum block there is the temperature sensor (KTY 81-110) that allows permanent check of the transistors temperature and that will lead to a warning on the LCD when excess temperature is detected.
The output transformer can be found under the two PA transistors and therefore is not visible here. This “stacked” construction saves very much space. PA transistors are connecting to 2.54 mm socket strips which makes the pair of semiconductors removable and allows access to the power transformer underneath.
On the right of the PA section there are the low pass filters for each band switched by a single relay.
Band filters are shared for transmitter and receiver and are switched to the respective branch by using relays. Left of the BPF unit there is a logical unit (HCF4028 BCD encoder and an ULN 2003 relay driver integrated circuit). This allows switching 5 relays by just using 3 binary coded controller output ports.
Software is written in C for AVR controllers using the GNU C compiler under Linux. The code will be discussed in the respective article that is going to follow this introduction.
I strongly recommend to stay tuned for the next articles covering this transceiver and giving details for each unit! 😉
The well-known mixer NE612 (NXP) will be compared to an AN612 (Matsushita/Panasonic) mixer that has been unsoldered from an old CB-SSB-radio. Comparison will include output voltage level and spectroscopic analysis of a 9MHz SSB signal.
When we talk about about integrated double balanced mixers (DBM) and say the number “612” we usually talk about the NE612 (aka SA/NE/602/612 in free combination of letters and digits). This IC uses a so called “Gilbert Cell” and has been developed by Dutch manufacturer Philips (nowadays NXP) some 30 years ago.
The IC has been intended to be used in cellphone applications, is a low voltage device (6 to 7V VDD approx., 8V DC max.) and has low power consumption . Frequency range is up to 500MHz (input signal) and gain is around 12 to 15dB. It has an integrated oscillator circuit that can be used with crystals connected to PIN6.
The IC has been widely adopted by amateur radio constructors and is still available today mainly in SMD package. When we examine homemade QRP radios published on the internet e. g., in 90% of cases one or more NE602 mixers will be found in the transceivers. One real advantage of the NE612 family is that only a few external components are required for building up a relatively acceptable working rf mixer.
In my radios I usually use the NE602 and its equivalents therefore for the DSB generator circuit and the transmit mixer. For receiving purposes it can be used for the higher bands (f >= 14MHz), on the lower bands the relatively low IMD performance (IMD3 about 15dB) shows severe shortcomings particularly on the 40 meter band where strong off-band broadcaster generate high signal levels and therefore overdriving the mixer’s input stage.
Due to the low IMD performance the IC also has weaknesses when being used as a DSB generator. The following findings occured when I analyzed the spectrum of a simple DSB/SSB generator equipped with an NE602.
NE612 DSB generator circuit under test
The NE612 here has been equipped with an additional resistor network (2x56k and a var. resistor with 10k) to get better carrier suppression features. To enhance output a transformer has been added to use PINs 4 and 5 which are the output stages of the circuit.
When driven with an dual tone audio signal (the 2 frequencies not harmonically related) we get an output voltage of about 50mV pp. and the spectrum shown below:
We can observe some IMD 3 and 5 products about 30dB below peak voltage. This is an outcome a little away from what can be expected from an SSB generator.
AN612 also is a very simple mixer that has been developed by Matsushita (Japan, now Panasonic) and has been used in various types of SSB radios for the 11m-Band (CB). In contrast to NE612 it does not contain an internal oscillator.
The IC comes in a 7 lead IC case (SIP7), please refer to datasheet. The IC is manufactured still today and available from various vendors on the internet. I ordered a package of ten from a Chinese ebay seller and found the ICs worked the same way like an original one from a PRESIDENT CB radio. They actually were no fakes.
The IC has a higher VDD so that it can be connected directly to the 12V rail of a standard battery operated radio. In contrast to the NE612 there is no need for a voltage regulator. Also the whole circuit only needs 7 external components:
Performance is quite interesting. When comparing this circuit to the NE612 DSB generator, we find that the output voltage is 4 times higher than that of its namesake. It equals to 200mV pp. The output spectrum also has slightly improved concerning IMD performance:
We see a little fewer IMD products with slightly decreased signal strength.
The AN612 is a not very well known but so much the better interesting mixer IC for the ambitious radio designer who wants to build hardware defined radios. The main locations in a radio will be the DSB generator and the transmit mixer. The IC is cheap, very well available and reveals a slightly higher performance than the other “612”, the NE612. And, overall, the circuit is very simple.
After having built this respective board with two NE612 ICs (one for DSB generator, one for the TX mixer) I was not satisfied with carrier suppression of the DSB generator. It turned out as only 40dB. Afterwards I constructed a new board with an old SIEMENS Mixer IC (S 042 P) that is still available NOS from various sources. With this one I gained carrier suppression rates of around 55dB. I think this is OK for a homemade transceiver.
The board looks as follows, set up on a 6x4cm 0.1″ veroboard:
The circuit starts with an AF amplifier equipped with a bipolar transistor where also a power supply for Electret microphones has been added. The radio now can handle dynamic and Electret microphones adequately.
Afterwards we see the S042P mixer IC where I have changed the circuit slighty to the one used in my 40-meter-QRO TRX. Audio input signal is now to PIN8 of the IC, Lo input on the rf side of the IC to PIN11 and PIN13. To reduce carrier level and enhance carrier suppression a 5.6pF cap is in series because the relatively high level of signal coming from the LO amp would deteriorate the performance of the DSB generator without countermeasures.
Output from this DSB generator is also symmetric and fairly high. Thus a low valued capacitor has been inserted prior to the SSB filter, sited on the RX board.
After that we see an amplifier with limited gain due to high emitter degeneration and the NE612 as TX mixer. The latter one also with an symmetric output to get more gain from it by using the two inherent output transistors.
TX-power amplifier stages
As I have described in the article of my “Give me 5“-Transceiver some years ago, building a broadband power amplifier is challenging due to one special problem related with the wide range of frequencies that this amplifier must be able to cope with. an extra gain of 5 to 6 dB is commen, when the frequency is divided by the factor of 2. Usually the necessary compensation is done by adding adequate capacitors and inductances using their frequency depending reactance.
With this radio I tried something new. I added an amplifier that is gain controlled by an adjustable voltage. Here a dual-gate MOSFET with gain control to gate 2 sets up the initial stage of the whole amplifier strip. The stage’s gain is set by a simple bipolar driver transistor controlled by a digital-analog-converter (DAC). A numeric value for each individual band is stored with in the EEPROM of the MUC. This numeric value is calculated during adjustment, then stored in the MUC and recalled whenever the radio is switched to a certain band. The DAC is an MCP4725 breakout board, containing a 12-bit device.
After that we see an amplifier that is common solid state technology. Preamp stage and predriver stage are set to A mode which requires a heat sink for the predriver stage. Here a 2N3866 is used as amplifying element.
Driver stage is single ended, operates in AB-mode and also is protected by a heat sink.
After that a somehow uncommon technique has been applied. Instead of using a broadband transformer to reduce the stages output impedance to the some ohms input impedance of the final stage, a set of 6 switchable low-pass-filters is used.
This filter section has been optimized to an output impedance of 50 ohms for each band thus enabling me to test and optimize the transmitter to a maximum with a defined output impedance (remember, this is an experimental radio! 😉 ).
After this filter section the final amplifier stage follows which is able to drive the output power up to 15 to 20 watts on all bands but depending on the DC voltage used for transmitting. The max. power gained during tests was 22 watts pep at 15V DC with two NTE236 transistors. Unfortunately the turned out not to be so rugged and blew in the tests. The eleflow 2SC1969 inserted later showed no problems at all. Thank God! When running on 12.0 V DC the amplifier puts out 12 watts at all bands.
The final part of the transmitter section is the last low-pass filter that is positioned next to antenna relay in the same compartment:
The whole transmitter looks like this:
The various units are:
1: DSB-Generator and TX mixer
2: Amplifier stages 1 to 4
3: MCP4725 transmitter gain controller
4: Intermediate LPF board
5: Power amplifier
6: Final LPF section
7: TX/RX switch board
Here a little bit of analysis to end with the article. First is the output of the SSB-Generator/TX-mixer board with maximum output (Around 500mV pp) set to the 40m band.
Nest we see the carrier suppression when dual tone audio in has been suspended. Carrier is about 55db under the signal peak.
And here an output signal with max. power at 3.5 and 7 MHz:
So, that’s all for today, thanks for watching and 73!
The receiver had to match a lot of requirements that should be described first:
Particularly on the lower bands and with effective long wire antennas the receiver front end will see high signal levels that it has to cope with. IMD always is a serious topic in this case.
Sensitivity particularly on the higher bands, where noise level is ow and signals are weak, is also an issue.
Dynamic range and extensive AGC gain compensation should be as high as possible.
This lead to a circuit that has proven its stability in lots of my radios:
Band filtering for each band with a double and loosely coupled LC circuits
Dual-Gate MOSFET (part of the AGC chain) as the first amplifier
Diode ring mixer (with Schottky diodes)
Post mixer amplifier with Dual-Gate MOSFET (part of the AGC chain)
SSB Filter (now 10.7 MHz) also used for transmitter (relay switched)
Main IF amplifier with MC1350 (part of the AGC chain)
Audio preamp with bipolar transistor
Audio final amp: (once again! 😉 ) LM386
Before describing the receiver itself we will have look at the band pass filter unit, that is shared between receiver and transmitter:
To minimize stray energy traveling from the input to the output of the filter, two SMD relays have been used on each side of the filter per band. And to reduce feedback fromt the transmitter (when the BPF is used to filter the TX signal after the TX mixer) the filter has been placed far away from the TX amplifier section.With an overwhelming result: The transmitter is nearly unconditionally stable now (compared to the TX section used in the “Give me 5”-Transceiver that had severe shortcoming in this aspect.
Control leads for the relays follow a designated coding scheme:
The receiver’s circuit
VFO signal is coupled into the DBM via a 10nF capacitor. The same is valid for the amplified RF signal from the output of the first amplifier stage using a Dual-Gate MOSFET (40676, BF900 or equ.).
Another Dual-Gate MOSFET is used as the post-mixer amplifier. All Dual-Gate MOSFETs so far are part of the AGC-Chain. This maximizes the possible gain swing to about 40 to 50 db. and enhances the receiver’s capability to handle even the strongest signal levels without distorting the output signal and the end of the audio chain.
Next is the SSB-Filter. Due to this is an “experimental” transceiver, the filter has not been soldered to the circuit board. Instead it is fixed with an aluminum clamp into two parts of header strips. Thus I can compare numerous SSB-Filters (9-, 10.695-, 10.7-MHz commercial ones, various home made ladder filters etc.). Here the different performance is very interesting to be explored.
The filter is accompanied by a special rf relay (manufacturer “Teledyne” with excellent performance concerning separation for the two channels) so that it can be used as the SSB filter for the transmitter section.
After the filter section the IF amplifier follows. This one uses an MC1350 video amp (old but good and still available, even in SMD!) and this IC also is controlled by AGC. The input is unbalanced (PIN6 to GND) the output is balanced and terminated with a tuned circuit.
Demodulator is an SA602 mixer IC.
After that the signal is handed over to the audio chain. But before the signal is processed in the next stage the frequency range is limited by a low-pass filter to reduce hiss. This filter also has two switched capacitors (controlled by MCU via NPN-driver stages) to adapt the sound to the preferred settings of the user. The software contains a respective function.
The audio amplifier consists of two sections: A preamp with a bipolar transistor and the inevitable and well-know LM386.
The full circuit on a 6×8 cm veroboard:
Starting from left top corner there is a 1:4 input transformer (not in the schematic), the preamp, the DBM, post mixer amp, SSB filter, relay, MC1350 as IF amp, demodulator and 2 stages of audio amp.
Performance is excellent. The circuit has no problem with high signal levels (in-band and out-of-band) especially on 40 meters. No IMD problems are noticeable even when used with high gain antennas like a 2×25 meter doublet with a tuner. On the higher bands noise figure is pretty OK what I think is based on the usage of Dual-Gate MOSFETs in 2 of the 3 amplifier stages. The MC1350 deteriorates this to a certain degree but is still very much acceptable for a shortwave radio.
The two DDS oscillators are mounted to the side of the cabinet. They are sited close to the microcontroller board to keep leads short.
Right on the left you can see the small dual-tone oscillator for testing and tuning. Next is the AD9834-equipped local oscillator (LO), centered the AD9951 that serves as the VFO. Right the ATmega128, mounted to a 64 lead breakout board can bee spotted behind the varios cables going to and from this section.
The Dual-Tone Oscillator
This one consists of two simple phase-shift audio oscillators. I have introduced this circuit a longer time ago for testing purposes here in this blog.
The capacitors and resistors in the phase-shifting chain have been chosen to put the two different frequencies to values of about 700Hz and 1900Hz, thus they are not harmonically related. A variable resistors allows the user to set the balance between the two signals so that they are equal in voltage.
Two transistors (a PNP-NPN pair) are switched by Pin PB7 from the microcontroller. There is a respective function in the software that activates the transmitter together with this oscillator for comfortable tuning and testing.
The Local Oscillator (LO)
This one again uses the “good old” AD9834, overclocked to 100MHz. I found that some chips from the “grey market” have problems when being overclocked and therefore produce spurious signals. In case this occurs, it is recommended to step back to the clock frequency of 75 MHz which is high enough for the purpose of the LO.
The oscillator comes with an balun output transformer (will reduce spurs!) and a low-pass filter plus a simple amplifier. The latter basically is not necessary because the LO will only have to drive the inputs of SA602 integrated mixer circuits (200mV RMS) used as SSB generator and rx demodulator. I had another mixer type in mind before, that one needed higher voltage. Thus the coupling to PIN6 of SA602 is only via 5.6pF capacitor to avoid overdriving the mixer and improve signal purity. This will be shown later when we are about to discuss receiver and transmitter circuitry.
Here the AD9951 DDS again comes to operation. This one has got a 14-bit DAC which makes it less prone for spurious signals. The clock rate has been pushed to the limit of 400MHz which, according to datasheet, is the max. clock rate for this DDS module.
You can download a datasheet of a suitable clock oscillator. This device is very small but it can be soldered to a 2 by 2 hole piece of veroboard and then mounted to a piece of headerstrip by soldering wires to the underside of the board:
A voltage divider will reduce the 3.3 V to 1.7V that is acceptable for the clock input of the AD9951 chip.
The DDS circuit is common for frequent readers of this blog:
The low pass filter has been left out because when examining the output signal of the DDS it turned out to contain only very little quantum of harmonics. The max. frequency of this VFO will be 29.7 MHz + 9MHz which equals to 38,7 MHz.
An SSB radio for the HF bands will be presented. Featuring 12 to 20 Watts of output power (depending on DC supply), full DDS frequency generation, covering 6 major frequency bands (1.8, 3.5, 7, 14, 21 and 28 MHz) within the short wave amateur radio spectrum. The rig also features colored LCD and front panel backlight.
In this upcoming series of articles a relatively complex project will be discussed. It is some sort of „remake“ of my last multi-band QRP SSB transceiver that has been entitled the „Gimme Five“-Transceiver and that was finished in 2015. „5“ in that case stands for the 5 major (i. e. „classical“) RF bands: 80m, 40, 20m, 15m and 10m the radio covered. This new project (called the „Midi6“, because it is not a “Micro” or a “Mini” transceiver 😉 ) covers one band more, the range has been extended to 160m.
The basic features of this construction are:
Dual DDS frequency generation (AD9951 as VFO, AD9834 as LO),
Colored LCD (CP11003) with resolution 240×320 pixels,
Single conversion superhet receiver, interfrequency 9 MHz,
5 stage high quality transmitter, Pout=20W (max. at 15V DC) , featuring a microcontroller driven regulated gain stage to ensure absolute constant output on all bands,
Integrated 2-tone oscillator for testing and tuning,
Front panel full backlight.
“Experimental radio” means that there is enough space inside the cabinet to change boards and test new ideas in the same space. Also certain components like the SSB-filter have been made as “plug-in” components to enable quick change of the part. Also the connector between the various transmitter and receiver stages have been done by “jumpers” and header strips so that resistors and capacitors can be changed quickly to experiment with other values.
The radio has been realized with standard veroboards (0.1″ pitch), SMD components and been put into a homemade aluminum cabinet using 2 layer sandwich construction inside the cabinet.
Here a snapshot of the operational transceiver. Cabinet size, by the way, is 7.5 x 16.5 x 19.5 centimeters (2.95 x 6.5 x 7.68 inches). These dimensions are in the range of other multiband QRP transceivers like the Elecraft K2 (larger) or the Icom IC703 (a bit smaller).
A general and good practice in engineering is a steady process of improvement. This article describes the construction of a high performance transmitter/receiver for SSB (voice) communication covering the 14MHz (20 meters) high frequency amateur radio band.
Various modules that have proven high performance, liability and ruggedness in recent constructions will be combined to form a radio with outstanding receiver performance, an ultra linear transmitter with output range 15 to 20 watts and a top audio sound quality both on transmit and receive.
Key features are:
Dual DDS frequency generation with AD9834 (Local oscillator) and AD9951 (VFO),
Microcontroller (MCU): ATMega644P by ATMEL,
Single conversion superhet receiver with 9MHz interfrequency (IF) and preamplifier, mixer and IF amplifier equipped with Dual-Gate-MOSFETs,
Audio-derived automatic gain control (AGC),
Transmitter with MC1496 as double sideband (DSB) modulator and NE602 as transmit mixer,
power transimitter with 4 stages, final stage in push-pull mode.
Another version of this radio has been built before. But this one was equipped with a variable frequency oscillator (VFO) because of nostalgia reasons. Unfortunately a VFO lacks certain features (frequency stability above all) which can be overcome by using digital frequency synthesis without losing performance. Usage of a high performance DDS systems is a prerequisite to achievement and a possible solution.
Most building blocks of that respective radio have been redesigned except the VFO section that turned out as not being able to deliver the projected frequency stability to a 100% degree. Frequency instability occurred because of the flatness of the former cabinet that brought the aluminum case too close to the VFO tuned LC circuit. Aluminum has a huge tendency to expand under the influence of heat so the rig was very temperature sensitive. That undeniable fault lead to a complete reconstruction using the old RX and TX modules and building a new set of frequency generators.
Parts of the old cabinet were reused but because of the fact that the whole rig got increased vertical expansion, the cabinet was “stretched” with two lateral strips of Aluminum.
Also a full electronic transmit/receive switch with p-channel power MOSFETs has been designed to avoid usage of a DC switch relay and get a “smooth” switching.
Another objective of this radio was to get out the absolute best performing circuits of the recent projects and to build a real high-performance radio. Hence this transceiver is also some sort of an improvement of the “Old school SSB TRX” as well. The circuit empirically turned out to be very good for communication in the 14MHz band. Because of this frequent readers of this website might detect certain similarities. 😉
The Receiver section
The design objectives were:
Low noise (achieved by using Dual-Gate-MOSFETs with the receiver to a large extent)
High dynamic range (achieved by using a Dual-Gate-MOSFET as receive mixer)
High AGC range (achieved by taking RF preamp and IF amp into the AGC chain)
Good audio quality (achieved by using a TBA820M as integrated AF amplifier circuit and a 5 cm loudspeaker)
RF preamplifier and receive mixer
The radio frequency preamplifier has been designed primarily to improve the receiver’s noise figure. Delivering additional gain only is relevant in second order.
Preselection is performed with only one tuned circuit int G1 line. The center frequency of this circuit is 14.180MHz. In the output section of the stage an another identical LC circuit has been installed. This turned out to be sufficient because there is no immediate need of higher preselection. The subsequently placed mixer, that is also equipped with a Dual-Gate-MOSFET has very good high level processing qualities. No interfrequency feedthrough could be observed with various antennas. No IMD occured even when signals were very strong. Testing out in the field with partable antenna very far from man-made noise sources the receiver was very quiet and even very weak stations could be received and read with Q5.
To get most of gain swing from AGC the preamplifier is controlled by a DC voltage between 0 and 12V supplied by the AGC control stage to be described later. This voltage is halved by a 1:1 resistor voltage divider because maximum gain of the Dual-Gate-MOSFET occurs with about 6V DC applied to G2.
Clipping diodes that are sometimes used to prevent high voltage entering the 1st stage have not been installed because they are prone to produce unwanted IMD products if signal levels from the antenna are too high and undesired mixing takes place there.
To prevent self-oscillation in the preamplifier, the tuned circuit LC1 and LC2 are connected together in a special way. G1 is connected to the tuned section of LC1. This section has high impedance, thus it should be connected to a load which also has high impedance. The coupling section of the coil with low impedance is connected to the 50Ω antenna. There are not two tuned parts of the LC circuits together in one stage.
The output of the Dual-Gate-MOSFET (low impedance) is connected to the coupling winding, the high impedance tuned part is going the high impedance of G1 of the mixer. The impedance ratio between the two coils is 16:4 due to the winding ratio of 4:1 of the coil set.
The sensitivity and noise figure of the whole receiver is determined by these two stages. Measurements showed that the minimum discernable signal is about 0.1µV which is very good for a short wave receiver.
SSB-Filter, IF amplifier, Demodulator, AF amp and AGC
The following stages are some sort of best practice combination of circuits that have proven to perform very well in the recent projects.
SSB-Filter and relay
The SSB filter is switched with a special rf relay by Teledyne® ensuring excellent isolation of relay ports with very low capacities in the unswitched signal path. Here the usage of shielded cable is mandatory for connecting the relay/filter section to the transmitter (see later text!). A clamp diode has been installed to eliminate high voltage peaks due to self-induction when the relay is switched. This will prevent the MOSFETs in the switching unit from excessive voltage and possible destruction.
A proven and reliable circuit can be found here as well. One stage delivers IF gain of about 12dB which is sufficient because the mixer following as a demodulator (NE612) also propduces some dB of gain. Too much gain in this section only contributes to high noise in the speaker later and is not desirable.
The Dual-Gate-MOSFET in this stage is also integrated to the AGC chain. Together with the RF preamp installed in the front and also being part of AGC control end we will get some 20 to 25 dB of gain swing when AGC is fully driven. This turned out to be enough, only in some rare cases I found that the manual gain control (also included in this recevier) needs to be used in addition when AGC is not able to cope with excessive signal levels.
Compared to a MC1350 IC equipped IF amplifier I found that gain control is much smoother because the V->dB function is very much less precipitously with the Dual-Gate-MOSFET than it is with the MC1350.
NE612 is built-in here. The main advantage of this IC is that it requires only a few components and it has got an additional gain of about 12dB or more.
In VDD line you will find a 5.6V Zener to bring 12..14V supply voltage down to about 6V. There are also two capacitors. The 0.1uF is for bleeding off rf energy from or to the supply rail, the same is the purpose of the 10uF cap for audio frequencies or low frequency noise present on VDD line. This noise sometimes originates from the digital components in the radio and should be eliminated at all reasonable points in the circuit. Also it will help to prevent the high gain amplifier chain from self-oscillating in the audio frequency range.
Audio frequency amplifier section
Two ICs are used here. The first is an operational amplifier (uA741) with a 150kΩ resistor as part of negative feedback circuit. This value is comparatively low. If (in rare cases) higher gain should be needed it can be replaced by e. g. 330kΩ or even more.
The main audio amp is the TBA820M, an integrated audio amplifier in 8 pin DIL case. It is an interesting alternative for LM386 because tendency for self-ocillation is much lower within the TBA820M. But it requires some more components. TBA820M can be switched with the load (speaker) to VVD or GND. I use a headphone jack in the radio, that is grounded, hence I prefer the latter version.
A “good” loudspeaker with 5cm of diameter was found by ordering a larger series of different speakers from Chinese vendors via ebay. The differences in sound quality are breath-taking. So it is worthwhile spending some money and order a larger variety of speakers and install the very best one.
This is a circuit I have used many times and it has proven to work very reliable. If you wish to have different settings concerning attack and decay time then another cap can be added via a switch to GND in parallel to the 47uF cap. Another 100uF for example will give a few extra fractions of a second in attack/decay time.
A 20kΩ variable resistor is used for manual gain setting. The AGC voltage that is near to VDD (12V or more) is divided and so AGC and manual gain control can be combined. At least until the point where noch AGCing will take place because the resulting voltage is <3V.
The “AGC thres.” variable resistor shown in the schematic will determine the point where AGC becomes active. I usually set it that way that solely band noise does not affect the AGC. Stronger stations (coming with S5 or 6 with a commercial transceiver) should give first minor influence on the AGC voltage. That is the point where amp gain should start dropping gradually. Strong stations must set AGC voltage to nearly 0 V.
The Transmitter section
The transmitter generally consists of two parts:
The SSB generator and the TX mixer, and
the Power Amplifier.
The full schematic of the two parts together:
Starting from the left we see the microphone amp. A nostalgic but still available operational amplifier integrated circuit (741) is used here. The amp has high gain (about 30dB) to make a dynamic microphone connectable. There is no DC feeding for an electret microphone. If you should wish to use one then the negative feedback resistors should be lowered to about 47kΩ and the audio level should be carefully observed to avoid excessive driving. DC must also be applied for htis type of microphone!
Double sideband generator
The MC1496 (still available as NOS in 14 pin DIL case or fresh from the market in SMD package by ON Semiconductors) offers high carrier suppression of about 50 to 60 dB. Therefore a network of 2 x 10kΩ and a 50kΩ variable resistor has been installed. The crucial point: To make full usage of this network the carrier offset must be set properly. If you should notice that there is no point within the full swing of the 50kΩ variable resistor then the carrier frequency should be readjusted.
A balanced output transformer has been installed to improve carrier suppression and to enhance output voltage.
SSB filter coupling out
The usage of shielded cable is mandatory here to avoid transfer of rf stray energy into the DSB and SSB line!
This stage also is equipped with an NE612 doubly balanced mixer due to reasons of circuit simplicity.
14MHz Band pass filter
This filter also needs observation. I use the TOKO style coil formers familiar from other projects. The winding data can be found in the schematic. The coil formers must have the ferrite caps and metal shield cans on to avoid incoupling of rf energy from the subsequent power stages. The filter should be placed away from the higher power stages to avoid self-oscillation inside the transmitter section.
RF amplifier power stages
The amplifier presented here has been tested in 2 different radios so far and has proven to be very stable, very linear and very rugged against antenna mismatch. The power levels are about 10 db gain per stage. From the second stage on the output impedance is 50Ω. This makes it easier to measure power levels with a 50Ω standard dummy load.
The 2 watt driver stage uses a PI-filter instead of a broadband transformer. This is because I intended to save some space on the veroboard and for a monoband transmitter this is a practical solution. If you should find out that there is a mismatch that results in losing gain, then the capacitors can slightly be modified because the L-network has impedance transforming capabilities. By knowing input versus output impedance and calculating a “Q”-factor subsequently L and C can be computed to get a defined step-down impedance (Link for further information). This is a useful method and, in case of low pass filter like applied here, there is also a filter for harmonics.
Driver and PA power amp are biased for AB-mode, all other stages operate in A-mode to ensure best linearity. Strategies using emitter degeneration and negative feedback are inherent in preamp and predriver stage.
All transistors apart from preamp stage require usage of heat sinks.
Impedance matching is either not done (stage 1 to 2), by transformer (stage 2 to 3) or by L-network (stage 3 to 4). Whereas from stage 3 to 4 also there is a transformer applied to split the signal symmetrically to the two bases of the final transistors.
If there should be a tendency for self-oscillation within this stage the input transformer secondary winding can be center tapped and put to GND via a 0.1 capacitor.
Power out depends on DC power voltage and is about 20 watts when run on 13.5 V DC power supply an the amplifier terminated to a 50Ω load.
This is a spectroscopical analysis of the fully driven transmitter (f=14.200kHz, Pout = 20.1 watts, VDD=13.0V) and the remaining carrier.
Harmonics are filtered very effectively . This is achieved by using a push-pull final stage driven in AB mode. Some authors say this is useful to eliminate odd number harmonics. On the other hand there are two sections of low pass filtering (one between driver and PA, one following PA). The figure of the output spectrum between and 50 MHz below:
The Dual DDS Oscillator System
The DDS has got the following features:
VFO: AD9951 + amplifier,
LO: AD9834 + amplifier,
LCD: NOKIA 5110,
Tuning: Optical rotary encoder by Bourns,
User interface: 4 keys to control the digital settings,
Analog inputs: User keys (ADC0), VDD (ADC1), S-Value (ADC2), TX PWR (ADC3), PA Temp. (ADC4).
The control lines for DDS1 (AD9951) and DDS2 (Ad9834) are as follows:
The colors are the colors used for the cables in my radio.
The LCD is connected likewise:
The NOKIA5110 LCD has been designed for VDD=3.3V. Please use 10kΩ resistors in the control lines which are not in the schematic! 3.3V are derived more or less closely by switching 2 Si-Diodes in series which results in a voltage drop of about 1.4V. Hence the LCD gets 3.6V DC from the 5V supply chain which is no problem for the module. One big advantage of the Nokia LCD should not be forgotten: It is very quiet and does not produce any discernable digital noise. Thus it is my favourite meanwhile for receivers on the RF bands.
For both DDS modules coupling out the rf is done with symmetrical circuits using trifilar broadband transformers. 10 turns on a FT37-43 core are a good choice. This will enhance gain and reduce spurs.
DDS2 is clocked to 110MHz, but keep in mind, that AD9834 is specified for 75 MHz max. clock rate only. I found out that modules from the “grey market” sometimes fail and produce lousy signals when overclocked. You can see that on a scope when extra peaks appear or with the spectrum analyzer when spurious signal are frequent. I recommend buying with Mouser or anther trusty vendor for example or reduce clock rate in case of problems in signal quality.
Power consumption is not excessive because both DDS modules are for low power application, unlike the AD9850 or AD9835, that draw much higher current. Power rate is 300mA when in receive mode with LCD backlight on.
The C-code for the software has about 2600 lines source code and can be downloaded here.
A standard CB DC supply cable is used here. Unfortunatel the plugs equipped with a cable and fuse holder are widely availabe but the sockets have to be stripped from old CB trasnceivers.
On the air the transceiver performs great. Audio is clear and powerful what the QSO partners often tell me. The receiver is fun to listen to, sounding soft AND precise. Maybe I will do a YouTube video the next weeks to prove it! 😉
All my rigs are for portable, hiking, bicycle trips and travel to foreign countries. I use Aluminum as a basis for the hardware to keep the radio lightweight. With this radio a ground plane made of 0.8mm Aluminum sheet metal has been used that one has been enforced with a lateral additional ground plane carrying the DDS system (see pictures in this article, please). Thus the base frame is pretty rigid and not prone for bending.
Front an rear panel are made from 0,.8mm Al sheets (rear) and 0.5 mm Al sheet (front).
The various subassemblies (DDS, receiver, transmitter) are split into different modules and are seperatelay fixed with bolts and washers mounted to special spacer bolts for screws of 2mm diameter. This ensures better grounding instead of using larger veroboards. Connections are made from flexible stranded hook-up wire and shielded cable for rf and audio signals.
On the undersides of the single boards copper foil is used for lines with GND portential.